Reinforced composite materials (or “prepregs”) have conventionally been formed from fibers (e.g., carbon or glass fibers) that are impregnated within a resinous matrix. Thermoset resins (e.g., unsaturated polyester, epoxy, or polyimide) are often employed as the matrix, particularly in applications requiring a high level of strength. One problem with prepregs formed from thermoset resins, however, is that they are generally brittle and have poor impact resistance. Furthermore, the prepregs are often difficult to store due to the short shelf life of the resin. In an attempt to overcome these issues, efforts have recently been made develop thermoplastic prepregs. One such prepreg is formed from a thermoplastic resin and unidirectionally aligned continuous fibers. Such a prepreg has excellent performance in terms of modulus of elasticity and strength in the fiber axis direction. However, because such prepregs have anisotropic mechanical properties, multiple prepreg layers are required during use that are oriented in different direction(s). This inevitably causes an increase in the cost and thickness of the resulting part. Other attempts to solve the problem associated with thermoset prepregs involve the use of a thermoplastic resin and chopped fibers formed by cutting unidirectionally aligned strands. While such prepregs exhibit better isotropic strength properties, the maximum volume fraction of the reinforcing fiber is generally low, which results in relatively poor modulus of elasticity and strength. Furthermore, it is difficult to tailor the mechanical properties of the prepreg through manipulation of the volume of such chopped fibers.
As such, a need currently exists for a method of forming thermoplastic prepregs that allows for selective control of its mechanical properties depending on the particular application. A need also exists for thermoplastic prepregs that have isotropic mechanical properties.